Something a bit different today. One of the folks in the "NRPLUS" National Review group put out the following query:
For those of you who are not religious, what is the “genealogy” of your morals and what is your argument they should be universal?
My thumbnail response, at a length appropriate for Facebook:
If you take a reductionist/determinist view, morality is impossible. It's all (imagine me here waving my hands at the universe) just atoms and fields bumping into each other according to relatively well-understood physical laws. You can't argue that a particular grouping of atoms is acting "badly", compared to another group of atoms that's being "virtuous".
In order to talk morality, you need to assume certain bedrock conceptions of life/consciousness/free will. We don't know where those things come from, at least not yet. But I take them to be (literally) self-evident; if they didn't exist, we wouldn't even be thinking about them.
Given that, add in "human nature": basic facts about our biology, psychology, and how we act in groups. That determines the envelope of our possibilities and desires. Gradually, we learn how best to satisfy our needs, working within our families and tribes.
From there, it's... well, probably see "The Abolition of Man" by C. S. Lewis, especially the stuff at the end where he describes cultures widely separated in time and space coming up with remarkably similar moral codes. (I know Lewis was a Christian, but TAoM isn't particularly theistic.)
As I type, it has a few likes. So, win.
Note that there's nothing in there about "universal". My guess is that human morality wouldn't apply to Klingons.
And if I had it to do over again, I'd add in "environment" to the "basic facts" that constrain and determine human survival and flourishing.
I'm also pretty sure that this approach isn't unique to me, it's just my distillation of arguments I've found compelling from other (deeper) thinkers.